1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which carries out printing by using a liquid ejection head having pressure generating elements corresponding to a plurality of nozzles.
2. Description of the Related Art
In general, in an inkjet type recording apparatus (inkjet printer), ink droplets are ejected at prescribed timings, respectively, from the nozzles of the recording head, on the basis of the dot pattern data (also called “dot data” or “print data”) developed from image data for printing which has been input from a host computer, and printing is carried out by means of these respective ink droplets landing on and adhering to a recording medium, such as recording paper.
As a method of the recording head, for example, a method is known which ejects ink droplets by causing a change in the volume of pressure chambers (pressure generating chambers) connected to nozzles. In a recording head of this kind, a diaphragm which is elastically deformable in the outward direction is formed on a portion of the circumferential walls which demarcate the pressure chambers, and the volume of the pressure chambers is changed by causing this diaphragm to vibrate by means of pressure generating elements which are represented by piezoelectric elements.
Usually, a plurality of nozzles are formed in a recording head, and a pressure chamber and a piezoelectric element are provided for each nozzle. All of the piezoelectric elements are electrically connected in parallel between a common power supply and ground wire, and a switching element is electrically connected in series to each of the piezoelectric elements. Signals (drive waveform) for driving the piezoelectric elements are generated by a drive waveform generating circuit, and they are distributed and supplied selectively to the piezoelectric elements via power supply lines and switching elements. More specifically, when a prescribed switching element is selected and switched on, on the basis of the print data, then a drive waveform is applied to the piezoelectric element, via a power supply line, and an ink droplet is ejected from a prescribed nozzle corresponding to the piezoelectric element to which the drive waveform has been applied.
In an inkjet recording apparatus which uses piezoelectric elements as described above, generally, a common drive circuit system is adopted, which uses one common drive waveform that combines a plurality of drive waveform elements for ejecting a plurality of types of ink droplets of different ink volumes (for example, a large dots, a medium dot, and a small dot), the required waveform portion being applied selectively to each of the piezoelectric elements, by means of switching elements. This system has benefits in that, since a common drive waveform is applied simultaneously to a plurality of piezoelectric elements, there is no need to prepare drive waveform generating circuits individually for each of the piezoelectric elements, and therefore the number of high-voltage high-precision analogue circuits, and the number of wires, can be reduced.
On the other hand, in recent years, with the object of increasing printing speed, and the like, printers based on a line head system (or array system) have been proposed, in which a very large number of nozzles are prepared, ink being ejected simultaneously from a large number of nozzles in such a manner that printing is carried out at high speed. If the common drive circuit system described above is applied directly to a line type of recording head having a plurality of nozzles, then the load of each drive circuit is high, and therefore the drive waveform is distorted. Consequently, not only does ink ejection become instable, but also, the current passing through all of the piezoelectric elements exceeds the drive capability of the drive circuits, and the heat generated by this may possibly lead to break down of the circuits.
One technique for resolving a problem of this kind is a method which drives the ejection of the piezoelectric elements at staggered timings. However, there is a problem with this method in that it impairs the high-speed printing capability which is a characteristic feature of a line type of recording head.
As a further possible solution, Japanese Patent Application Publication No. 2005-59440 discloses a method (circuit division method) which divides a very large number of piezoelectric elements into a plurality of groups and then drives the piezoelectric elements by means of drive circuits which are separated according to the groups in such a manner that the drive circuits respectively drives the groups. According to this method, the load is divided between a plurality of drive circuits, and the required current and heat generation in each drive circuit is reduced. Therefore, the high-speed characteristics of the line head system can be utilized. However, although the composition described above is extremely beneficial with respect to the drive capacity, heat generation and power consumption of the drive circuits, it has the following problems. More specifically, in a line head system in which a plurality of nozzles are arranged in an array configuration, it is necessary to drive ejection of ink from a very large number of nozzles, simultaneously, and therefore, the momentary current consumption of the print system as a whole may become problematic, depending on the timing of the drive waveforms generated by the plurality of drive circuits. When a plurality of piezoelectric elements are driven simultaneously, a current flows momentarily, and therefore it is necessary to prepare a drive circuit having a high drive capacity, or an appropriate number of drive circuits, in order to handle a large current. In addition to this, excess capacity is required in the power source which supplies power to the drive circuits, and it is difficult to supply the power from a wall socket or distribution panel. Furthermore, even supposing that an extremely large power source can be prepared so that the prescribed power can be supplied to the drive circuits, not only does the output voltage of the actual power source fall momentarily as a result of the momentary current, but also, a voltage drop is caused by a pattern in the power lines from the power source to the piezoelectric elements, impedances of wiring such as FPCs, or the electronic components such as transistors and resistances. Therefore, the voltage of the drive waveform ultimately applied to the piezoelectric elements falls, and it becomes difficult to eject ink appropriately. Therefore, image quality is degraded.
A method for improving the aforementioned circuit division method is known, in which the momentary current and the momentary power are restricted by controlling the timings of the drive waveforms (in other words, the phases of the drive waveforms) from a plurality of drive circuits. However, it is difficult to restrict the momentary current simply by staggering the phases of the drive waveforms, and furthermore, the time taken for printing is lengthened in accordance with the amount by which the phases are staggered. Therefore, the high printing speed which is characteristic of a line head system is impaired. Moreover, displacement of the deposition positions (landing positions) of the ink droplets may occur, and this may be visible as displacement of the pixels.
Furthermore, in a recording head, there is some, albeit slight, variation in the nozzle shapes, and this may give rise to variation in the deposition positions of the ink droplets. In particular, in a recording head having a plurality of nozzles, such as a line head, variation in the deposition positions caused by variation in the nozzle shapes is unavoidable, and there is a high possibility that this leads to be visible in the form of non-uniformities in the image.